A 60-year-old woman presents to a neurologist with worsening morning headaches that are throbbing and accompanied by nausea. She reports that the headaches are positional and worsen when lying down. Additionally, she has noticed a decline in her sense of smell and has difficulty seeing out of her right eye. She denies pain or diplopia but has been experiencing left-sided clumsiness. She has a medical history significant for hypertension, hypercholesterolemia, and ischemic heart disease and takes aspirin, ramipril, and simvastatin. She smokes 10 cigarettes per day and drinks four units of alcohol per week. On examination, she has bilateral non-tender pulsatile temporal arteries and a sluggish direct pupillary reflex on the right with a relative afferent pupillary defect. Fundoscopy reveals right optic atrophy and mild swelling of the left optic disc with peripapillary hemorrhages. Visual field testing shows a central scotoma on the right and restriction of visual fields on the left. The rest of the cranial nerves are normal, and examination of the peripheral nervous system is unremarkable. Laboratory investigations are within normal limits except for a slightly elevated white cell count and a low ESR. What is the most likely diagnosis for this patient’s symptoms?